Method of treating raw cellulosic materials

ABSTRACT

METHOD OF TREATING RAW CELLULOSIC MATERIALS REQUIRING DIFFERENT PROCESS CONDITIONS IN A VERTICAL DIGESTER HAVING A VAPOR PHASE COOLING ZONE AND/OR A LIQUID PHASE COOKING ZONE, ONE OF THE RAW MATERIALS BEING FED INTO THE VAPOR PHASE COOKING ZONE FOR TREATMENT THEREIN FOR A PREDETERMINED PERIOD OF TIME PRIOR TO MOVING INTO THE LIQUID PHASE COOKING ZONE, THE OTHER RAW MATERIAL BEING FED DIRECTLY INTO THE LIQUID PAHSE COOKING ZONE IN THE FORM OF A CENTRAL CORE FREE OF DIRECT CONTACT WITH THE STEAM IN THE VAPOR PHASE ZONE DURING ITS PASSAGE THERETHROUGH.

METHOD OI TREATING RAW CELLULOSIC MATERIALS Filed April 10, 1972 United States Patent 3,785,920 METHOD OF TREATING RAW CELLULOSIC MATERIALS Thor H. Johansen, West Nyack, NY. American Defibrator, 405 Lexington Ave., New York, N.Y. 10017) Filed Apr. 10, 1972, Ser. No. 242,423 Int. Cl. D21c 3/26 U.S. Cl. 16219 4 Claims ABSTRACT OF THE DISCLOSURE Method of treating raw cellulosic materials requiring different processing conditions in a vertical digester having a vapor phase cooking zone and/or a liquor phase cooking zone, one of the raw materials being fed into the vapor phase cooking zone for treatment therein for a predetermined period of time prior to moving into the liquid phase cooking zone, the other raw material being fed directly into the liquid phase cooking zone in the form of a central core free of direct contact with the steam in the vapor phase zone during its passage therethrough.

BACKGROUND OF THE INVENTION This invention relates to a method of feeding cellulosic fibrous material in a continuous vertical digester.

In the method of producing pulp for paper products the fibrous lignocellulose material, such as pulp wood, after having been reduced to chips, is conducted through a steaming vessel where the chips are heated with steam under superatmospheric pressure, to expel air therefrom in order to facilitate the absorption of cooking liquor in the subsequent impregnating step. In the impregnating vessel, the chips are impregnated with a hot chemical liquor at an elevated temperature and pressure. The thoroughly impregnated chips, after excess liquor having been drained off, are introduced into a digester vessel, in which the chips are digested under steam pressure at temperatures of 3203 60 F. About three quarters of the vessels volume may be filled with cooking liquor. The digester is provided with steam inlets at its upper part. The cooking is carried out with saturated steam in vapor phase at the top of the vessel. The cooking time is determined by the chip level in the digester and the retention time allowed in the vessel depends upon the nature of the pulp material, temperature, liquor concentration and production rate.

A gamma ray source and a detection unit is used to sense the level of the chips in the digester. The chip level is in turn controlled by the speed of the discharger at the bottom of the digester vessel. The digester chips are fed out from the bottom of the digester through an Asplund Defibrator whereafter the resultant defiberized stock is blown into a cyclone in which it is diluted to the desired consistency, and then conveyed to a disc press for press ing out the liquor. After the disc press, the pulp falls down into a conveyer screw housing from which the pulp is conveyed for further treatment in washers, while pressed out spent liquor is drained to a black liquor tank from which cold liquor is pumped back to the digester to flash or cold blow the bottom of the vessel.

Since the cooking time and the retention time of the cellulosic material in the digester varies according to the type and size of the raw material, heretofore only one type of raw material could be successfully treated through the digester at one time. Thus, relatively large chips and softwood require longer cooking time than the relatively small chips and fines, such as sawdust and hardwood. It has been found that when these preimpregnated particles are treated in the digester, they should be introduced below the liquor level directly into the liquor phase of the "ice digester, as otherwise the vapor phase cooking would reduce them to a slurry, which not only would affect yield and quality of the pulp, but would tend to clog the screens at the steam and liquor inlets. Therefore, the actual pulping of small chips and fines, such as sawdust, should be restricted generally to the lower temperature in the liquor phase of the digester. Consequently, such type of raw material could not be treated in the conventional digester, together with raw material composed of larger and harder chips, which require considerable softening by cooking prior to contact with the liquor.

The present invention contemplates the solution of the aforesaid problem and the object thereof is to provide a method whereby raw materials requiring different reaction times in the digester can be fed simultaneously into a continuous digester.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic view of a digester system in which the process according to the invention is carried out.

SUMMARY OF THE INVENTION The invention resides in the method of feeding a second type of material requiring a different reaction cycle using a screw feeder arrangement feeding into an injection chamber provided with an impeller and connected directly to the liquor circulating line for the purpose of feeding said material into the center core of the wood chip column inside the vertical digester.

DESCRIPTION OF A PREFERRED METHOD OF CARRYING OUT THE INVENTION The raw material indicated at A is composed of relatively large chips or splinters from which the smaller chips and sawdust have been separated in accordance with conventional procedure, are transported to the hopper 1.

The relatively coarse chips fall down into a conventional rotary feeder or cell feeder 2. From the cell feeder, the chips are pumped together with circulating liquor from the tank 3 through the line 4 into a separator 5 in the dome of the digester, which is generally designated at 6. In the separator, they are conveyed upwardly to spill over the edges thereof into the top of the digester, while the circulating liquor is drained through perforations on the wall of the separator and returned to the tank 3 through line 11.

The vapor phase portion 7 of the digester is heated by injecting steam of superatmospheric pressure and corresponding temperatures, such as 320-360 F., through the steam lines 8.

In the vapor phase portion, the chips A are retained for a period of time depending upon the type of raw material, the degree of softening or steam cooking that is required. For instance, conditions for processing birch are considerably milder than for spruce. The time can be varied by adjusting the liquor level in the digester, as will hereinafter be explained. The hot steamed-cooked chips soak up liquor very rapidly when submerged in the cooler liquor in the liquor phase portion 12 of the digester, which explains one reason for the present invention.

The liquor level is controlled by the adjustable chip level gamma ray device 13.

The chips A moving down from the cooking or vapor phase zone are submerged in liquor in the liquor phase zone of the digester. Liquor from tank 3 circulates in the upper portion of the zone. The liquor supplied by the line 15 is withdrawn from the digester through the line 11 and conveyed to the tank 3 to be used as a propellant in conveying the chips A to the separator at the dome of the digester.

In order to prevent escape of pulp material from the digester, strainers or screens 14 are provided at the liquor outlets. Clogging of these screens or strainers has been a problem in treating finer chips and finely comminuted material, such as fines and sawdust. The present invention also solves this problem, as will be understood from the further explanations herein.

The retention time in the liquor phase zone may vary from -%-1 /2 hours, depending on the degree of cook required. Strainer design, vessel design and downward liquor circulation combine to assure positive and uninterrupted movement of the chip mass.

The liquor phase zone terminates in a cooling zone 18. The cooling zone uses liquor commonly referred to as cold blow" liquor from the black liquor tank, from which it is pumped through the heat exchanger 20 into the digester, in the line 21. The black liquor is drained from the screw conveyer 22 beneath the cyclone 23 into which the pulp is blown through the blow valve 24 after having been defiberized in the Asplund Defibrator 25. At the bottom of the digester, the mass of chips are fed to the Asplund Defibrator by means of the wing discharger 26.

The conveyer 22, transports the pulp to the disc press 27, from which it falls into the stock chest 28.

The foregoing procedure is conventional in processing chips of substantially uniform type requiring the same degree of cooking and retention time. Obviously, raw material, such as fines and finely comminuted material, such as sawdust, could not be fed together with, and be subjected to the same treatment and retention time in the digester without risk of serious loss of yield and quality of the pulp.

According to the invention, the steam heated material generally indicated as B, composed of relatively finely comminuted material, such as fines and sawdust, is transported to the hopper 29 where it is compressed by the screw feeder 30 into a plug, which is discharged into the vertical injection chamber 31 which is filled with impregnating or cooking liquor which is pumped from the white liquor tank 9. During the compression step most of the air trapped inside the small chips or particles is expelled. As the plug of small chips or particles is discharged into the liquor filled impregnation vessel, they expand and soak up the liquor in the manner of a sponge, and substantially regain their original volume. The soaking process may be accelerated by keeping the liquor temperature lower than that of the steam heated chips. The chips are carried upwards through the impregnating vessel and are kept in liquor suspension for a sufficient period of time to ensure thorough soaking. Eventual lumps are broken down by the impeller 34 before going through the strainer 35. Thus, the surfaces of the fine chips will be substantially maintained and preserved against fiber damage so as to provide a reaction surface for the liquor during the liquor phase treatment in the digester.

A valve 32 automatically serves to close the discharge opening of the conveyer 30 in the event of blow back caused by escaped pressurized liquor from the digester.

The thoroughly soaked mass of chips is introduced uniformly into the conduit 33 by means of the rotating impeller 34 and fed into the central pipe mounted below the separator 5. The pipe 36 confines the downwardly moving mass of chips and liquor within the constricted passage of the pipe, free of contact with the steam environment in the vapor phase zone of the digester, thus preserving it against fiber damage.

The pipe 36 extends slightly below the level of the liquid in the digester so that the material B will be introduced into the liquor in the form of a plug which moves downwardly in the liquor phase portion of the digester to form a central core for the material A. Thus, the core of fine particles will be confined by the surrounding coarser chips and prevent them from clogging the strainers or screens during their downward movement.

It should be noted that clogging up the screens, which permit circulation of liquor, would interfere with the uniform process of the mass of chips through the digester and seriously affect the selected reaction time in the liquor phase zone. Uniform temperature and proper digester balance are important factors for attaining uniform high yield pulp.

The foregoing description is given by way of example and not by Way of limitation. It should be understood that the invention may find a variety of expressions within the scope of the appended claims.

What is claimed:

1. The method of feeding cellulosic materials requiring different processing conditions in a digester having a vapor phase cooking zone and a liquor phase cooking zone, which comprises:

(a) feeding a mass of chips of a first material into the vapor phase zone of the digester for treatment therein in an environment of steam under superatmospheric pressure and at correspondingly elevated temperature for a predetermined period of time;

(b) moving the thus treated mass of chips into the liquor phase zone;

(c) feeding a mass of chips of a second material and of a smaller size than the said chips of the first material in the form of a plug directly into the liquor phase zone free of contact with steam in the vapor phase zone;

(d) moving said second material centrally through the liquor phase zone together with the mass of chips of the first material in the form of a core therein, and

(e) discharging the treated material from the digester.

2. Method according to claim 1 in which the mass of chips of the second material prior to being fed into the digester is compressed and subsequently expanded in a liquor filled injection chamber under superatmospheric conditions to form a slurry.

3. Method according to claim 1 in which the mass of chips of the second material is fed into a central pipe extending through the vapor phase zone of the digester into the liquid phase zone.

4. Method according to claim 2 in which the mass of chips of the second material is fed into a central pipe extending through the vapor phase zone of the digester into the liquid phase zone.

References Cited UNITED STATES PATENTS 3,554,864 l/l971 Richter 162237 3,578,554 5/1971 Richter 16252 3,579,418 5/1971 Ostberg 162-52 S. LEON BASHORE, Primary Examiner M. S. ALVO, Assistant Examiner U.S. c1. X.R. 162-42, 237, 246 

